In a finished liquid crystal flat panel, a thin layer of liquid crystal (LC) material is disposed between two plates of glass. On one plate of glass, one or more two-dimensional arrays of electrodes are patterned, each array referred to as a panel. Each electrode may be on the order of 100 microns in size and can have a unique voltage applied to it via multiplexing transistors positioned along the edge of the panel. In a finished product, the electric field created by each individual electrode couples into the LC material and modulates the amount of transmitted light in that pixilated region. This effect when taken in aggregate across the entire two dimensional array results in a visible image on the flat-panel.
A significant part of the manufacturing cost associated with liquid crystal display (LCD) panels occurs when the LC material is injected between the upper and lower glass plates. It is therefore important to identify and correct any image quality problems prior to this manufacturing step. The problem with inspecting LCD panels prior to deposition of the liquid crystal material is that without LC material, there is no visible image available to inspect. Prior to deposition of LC material, the only signal present at a given pixel is the electric field generated by the voltage on that pixel.
To overcome this limitation, Photon Dynamics has developed a floating modulator which, in part, includes a relatively large piece of optically flat glass with a thin layer of LC material formed on its surface, as shown in FIG. 1A. To inspect the patterned glass plate 10, modulator 15 whose dimensions are smaller than those of the flat panel, is physically moved over a portion of the panel to be inspected and then lowered to within a few microns of the flat panel's surface, as shown in FIG. 1B. A drive signal is applied to the electrodes on the panel. The small air gap 25, typically 10 to 50 micrometers, between the flat-panel electrodes 30 and the LC modulator 15 allows the electric field from each pixel electrode 30 on the patterned glass plate 10 to couple to modulator 15 to create a temporary visible display of the panel. This visible display is subsequently captured by camera 35 for identification of defects. After inspecting a region, modulator 15 is lifted and moved to another region on the panel and the process is repeated. Through this step-and-repeat process, the entire LC panel can be inspected for defects. In FIGS. 1A and 1B, LC modulator 15 is shown as including an LC material 45 and a flat glass 50.
Inspecting an LCD panel at high speeds using the modulator described above poses technical challenges. For example, when inspection at one site is completed, the modulator, which may weigh several pounds and which also lies very close to the panel during inspection, is first lifted to ensure that the modulator does not damage the glass panel, and then moved to the next site and lowered towards the panel for the next inspection operation. These movements plus any time required to allow settling of the movements impacts the system throughput. Presently known step-and-settle modulators do not readily lend themselves to continuous linear scanning, which may provide far higher system throughput, primarily because of their form factor which is far smaller than the large substrates.
With the modulator described above, the visible image of the thin LCD layer is obtained by reflecting light from the surface of the LC material. The LC material acts a scattering medium in its off-state and a transmissive medium in the on-state. This typically results in the generation of a DC-component of light modulated with a relatively small amount of information. To the camera 35, this means that the imager must be able to handle a relatively large signal (for the DC component) even though the signal containing the information is relatively weak. Furthermore, the relatively large DC-component of light component may carry a correspondingly large amount of shot noise which needs to be overcome to enable one to reproduce the flat panel defect data.
Another method of panel testing uses an electron beam and imaging apparatus to detect defects. Typical electron beam testers include several electron beam/imaging heads that step along the panel surface and requires that a drive signal is applied to the panels, as are found in the electro-optical modulator based tester. However, since the electron beam heads can be smaller in size, several electron beam heads may span across the width of a panel, and thus the amount of side-ways stepping can be less in the electron beam tool than a modulator-based tool. An electron beam based tool requires vacuum, and the electron beam sensor heads cannot fully span the width of a flat panel.